Generally, in visually observing an image of an object surface of an object with a microscope (for example, in the case where an image of an object surface formed on a focal plate by an objective lens is observed through an eyepiece), when the object surface exists within a focal depth of the objective lens, the image of the object surface formed on the focal plate appears in focus even if a relative positional relationship between the object surface and the objective lens is changed within the focal depth range in an optical axis direction of the objective lens. Therefore, in the case where a size and a shape are to be accurately measured in a certain object surface of the object, and where a size in a height direction of the object, for example, a distance between two object surfaces deviated in the optical axis direction of the objective lens, is to be accurately measured, a focusing apparatus is particularly required to adjust the relative positional relationship to match a focal position of the objective lens accurately with each object surface.
As a patent document prior to the captioned application, UK Patent No. GB2076176A (Patent Document 1), for instance, discloses a microscope provided with a focusing apparatus. FIG. 1 is a view showing a schematic configuration of a microscope equipped with a conventional focusing operation assist apparatus disclosed in Patent Document 1. A lens 12 is disposed such that a collector lens 10 and a split prism 5 are conjugate with each other. The split prism 5 includes a base prism 14 having a predetermined apex angle and a semicircular prism 16 having an apex angle which is twice as large as that of the base prism. A focused pattern 15 is formed on the light source side of the base prism 14. The focused pattern includes at least one line, and the focused pattern straddles two regions to which different deflection angles are respectively imparted by the base prism 14 and the semicircular prism 16.
The light emitted from the light source is incident to the split prism 5 while the luminous flux is restricted to an optimum condition for focusing assist, by an aperture diaphragm 13. An image of the aperture diaphragm 13 is formed on a diameter-variable microscope aperture diaphragm 19 through lenses 17 and 18. However, the image of the aperture diaphragm 13 is branched into two images by a deflection angle action of the split prism 5, and the two images are formed while deviated from each other in a symmetrical manner in relation to the optical axis of the focusing apparatus.
The two branched images of the aperture diaphragm 13 are formed on an objective lens 23 (strictly, on a pupil of objective lens 23) through a beam splitter 22 by image forming action of a lens 21.
The focusing assist apparatus is placed on a stage of a vertical motion device (not shown), an operation unit of the vertical motion device is operated to move the whole of the optical system in the optical axis direction so as to adjust a relative positional relationship between the objective lens 23 and an object surface 24 in the optical axis direction. The optical system is disposed such that it is observed within a visual field that the focused pattern images move in opposite directions when the operation unit of the vertical motion device is operated to move up and down the optical system, and such that the focused pattern images are observed to be matched with each other when a focal position of the objective lens 23 is matched with a specimen surface 24. In a non-focused state (for example, the state in which the specimen surface is located at a position indicated by numeral 24a of FIG. 1), the focused pattern images are observed while deviated from each other, and two lines of the focused pattern are projected away from each other by d on the specimen surface 24a. 
The aperture diaphragm 13 and the split prism 5 are placed so as to be inserted into and retracted from an optical path by an inserting and retracting mechanism (not shown). During a period of the focusing assist, the split prism 5 and the aperture diaphragm 13 are inserted into the optical path to match the focused patterns, thereby establishing the focusing. During a period of observation, the aperture diaphragm 13 and the split prism 5 are retracted from the optical path, so that the focusing assist apparatus acts as a usual microscope epi-illuminating device.
Patent Document 1: UK Patent No. GB2076176A
In the focusing assist apparatus disclosed in Patent Document 1, using the lenses 17 and 18, the aperture diaphragm 13 is disposed at a position which is conjugate with the microscope aperture diaphragm 19 while the focused pattern 15 is disposed at a position which is conjugate with the microscope field stop 20. The aperture diaphragm 13 and the focused pattern 15 are projected to the objective lens pupil plane and the specimen surface 24 after intermediate images formed, respectively. Therefore, the optical components 12 to 18 can be modularized and can be added to usual microscopes. At the same time, the apparatus configuration becomes complicated, which leads to higher costs and a larger size.
In view of the foregoing, a problem of the present invention is to provide a simple and compact microscope provided with a focusing assist apparatus in which focusing can accurately be performed with no restriction of magnification or NA of the objective lens.